In recent years, nondestructive sensing technology using a terahertz wave has been developed. The field of application of electromagnetic waves in this frequency band includes a technique of making a safety imaging and examining apparatus replacing a fluoroscope to perform imaging. Furthermore, a spectroscopic technique for obtaining the absorption spectrum or complex permittivity of a substance to examine physical properties, such as molecular bonds, a measurement technique for examining physical properties, such as carrier density, mobility, and conductivity, and an analysis technique for biomolecules have been developed. As regards a method of generating a terahertz wave, a method using a nonlinear optical crystal has been widely used. Typical examples of nonlinear optical crystals include, for example, LiNbOx (hereinafter, also referred to as “LN”), LiTaOx, NbTaOx, KTP, DAST, ZnTe, GaSe, GaP, and CdTe. To generate a terahertz wave, a second-order nonlinear phenomenon is used. A known process is difference frequency generation (DFG) caused by two incident laser beams having different frequencies. In addition, monochromatic terahertz wave generation based on an optical parametric process and a process of generating terahertz pulses via optical rectification caused by femtosecond pulsed laser irradiation are also known.
As for the method of generating a terahertz wave using such a nonlinear optical crystal, electro-optic Cherenkov radiation has recently received attention. This is a phenomenon in that when the propagation group velocity of a laser beam 91 as an excitation source is higher than the propagation phase velocity of a generated terahertz wave, the terahertz wave 92 in conical form is radiated like a shock wave as illustrated in FIG. 9. A radiation angle θc is determined on the basis of the ratio of the refractive index of a terahertz wave in a medium (nonlinear optical crystal) to that of light by the following equation:cos θc=vTHz/vg=ng/nTHz where vg denotes the group velocity of excitation light, ng indicates the group refractive index thereof, vTHz denotes the phase velocity of the terahertz wave, and nTHz indicates the refractive index thereof. A report has been published (refer to NPL 1) which describes that the Cherenkov radiation phenomenon is used and a femtosecond layer beam having a tilted wave front is allowed to enter LN to cause optical rectification, thus generating terahertz pulses of high strength. In addition, a report has been published (refer to NPL 2) which describes that a slab waveguide having a thickness enough smaller than the wavelength of a terahertz wave to be generated is used to save the need to tilt the wave front and a monochromatic terahertz wave is generated by DFG.
The cases in the above-described Non Patent Literature relate to a proposal in that terahertz wave generation is caused by traveling wave excitation and terahertz waves generated from different wave sources match in phase in the direction of radiation and thus enhance each other to improve extraction efficiency. As regards the characteristics of this radiation method, relatively high efficiency can be provided using a nonlinear optical crystal and terahertz waves of high strength can be generated. In addition, the frequency band of terahertz waves can be widened by selecting absorption in a terahertz region, caused by phonon resonance typical of crystal, on the high frequency side. These techniques allow the generation band to be wider than that in terahertz generation using a photoconductive element and the pulse width can be reduced when terahertz pulses are generated using optical rectification. For example, when these techniques are applied to a terahertz time-domain spectroscopy apparatus, the performance of the spectroscopy apparatus is expected to be improved.